The invention relates to an image sensor device. The image sensor device comprises a semiconductor substrate of a first conductivity type having a surface. A number of channel regions of the first conductivity type are formed at the surface of the substrate. The channels extend along the surface transverse to a system of electrodes present on the surface. During operation of the image sensor device, charge is collected and transported in the channel regions. The channel regions therefore are separated from each other by channel separation zones of a second conductivity type opposite to the first conductivity type. The channel separation zones are also formed at the substrate surface. Beneath the channel regions is a semiconductor zone of the second conductivity type extending substantially parallel to the substrate surface.
In such an image sensor device, during operation voltages are applied to the electrodes to form a pattern of potential wells separated by potential barriers in the channel regions. For a given integration time, charge produced in the semiconductor material by incident radiation is collected in these potential wells. Thus, a charge image corresponding to a radiation image is formed. After the integration time, clock voltages are applied to the electrodes to transport the charge packets through the channel regions to a storage register. Such a method is called frame transfer or field transfer. Subsequently, the charge is further processed during the next integration period to produce a television signal.
By the application of suitable voltages between the electrodes, the semiconductor zone and the substrate, a potential barrier can be produced in the semiconductor zone. Thus, charge which, viewed from the surface, is produced above this potential barrier in the semiconductor material will contribute to the formation of the charge image. Charge produced beneath this potential barrier will not contribute to the formation of the charge image. Since long wavelength radiation can penetrate more deeply into the semiconductor material than short wavelength radiation, the spectral sensitivity of the image sensor device is determined by the position of the potential barrier.
British Patent Application No. 2,054,961 discloses an image sensor device in which the semiconductor zone and the channel regions have doping concentrations which do not exceed that of the substrate. As a result, charge collected in the channel regions can influence the potential variation between the surface and the semiconductor substate so that the potential barrier, which was initially present at the area of the semiconductor zone, disappears when a given quantity of charge is exceeded during the integration period. When during the integration period, high intensity irradiation produces excessive charge, the excess charge can flow away to the semiconductor substrate. Thus, this excess charge will not be spread over a large number of adjacent potential wells present in the channel regions during the integration period. This spreading of charge, often designated as "blooming", can give rise to very disturbing lines in a television picture which is formed from signals produced with such an image sensor device.
The known image sensor device comprises a semiconductor substrate on which two semiconductor layers are disposed. The upper layer comprises the channel regions. Both layers have doping concentrations which do not exceed that of the semiconductor substrate. Such a construction cannot be obtained by diffusion of impurities in semiconductor materials. To form layers by diffusion of impurities into a semiconductor body of one conductivity type, a zone of the other conductivity type can be formed only by providing a dopant concentration which exceeds that of the semiconductor body. In order to be able to manufacture the known image sensor device, first a layer of the second conductivity type and then a layer of the first conductivity type will have to be grown epitaxially onto a semiconductor substrate. In this multilayer structure, channel separation zones extending into the lower of the two layers can be formed by means of diffusion of impurities in both layers.
Another disadvantage of the known image sensor device is that the channel regions have widths which are determined during the manufacture of the channel separation zones. The separation zones will have minimum widths which are equal to the minimum dimensions of the windows required for the diffusion, plus two times the distance over which lateral diffusion takes place. This diffusion distance is larger in the known image sensor device than the thickness of the layer in which the channel regions are formed. Starting from a desired center-to-center distance between adjacent channel regions, the desired center-to-center distance minus the width of the required diffusion window and well over twice the thickness of the channel regions is then left for the width of the channel regions. In practice, the desired center-to-center distance is, for example, 10 .mu.m, the width of the window is 4 .mu.m and the thickness of the channel regions is 1 .mu.m. The width of the channel regions is then only about 3 .mu.m.
The known image sensor device consequently has comparatively narrow channel regions and comparatively wide channel separation zones. This is undesirable because the quantity of charge that can be collected and transported per unit surface area is comparatively small, and also the image sensor device consequently has a comparatively low sensitivity. Charge produced in the channel separation zones flows away to the semiconductor substrate and does not contribute to the formation of the television signal.